1. Field of the Invention
This invention relates to solvent refining processes used in the production of oil products such as lubricating oils, and more particularly relates to an improved process for recovering solvent from a solution thereof.
In general, the present invention is broadly concerned with a variety of solvent recovery processes associated with the processing of a variety of crude hydrocarbon sources such as naturally occurring petroleum hydrocarbons, refined shale oils, tar-containing sands, etc.
The quality of various oil or petroleum fractions derived from such crude hydrocarbon sources has for a long time been upgraded by removal therefrom of certain undesirable constituents. For example, in the case of vacuum gas oils or deasphalted residual oils wherein such constituents, for example, comprise cyclic, naphthenic, olefinic, aromatic, nitrogenous, and sulfurous compounds, or mixtures of such compounds, such constituents are generally removed by a solvent refining or extraction process, utilizing selective solvents such as furfural, phenol, N-methyl pyrrolidone, nitrobenzene, and the like, thereby to increase the viscosity index and stability of the resulting product. However, in the case of wax-containing oils, wherein such constituents comprise materials having a high melting point such as waxes which impair either the pour point or cloud point of the resulting desired product, such constituents are generally removed by a solvent dewaxing process, utilizing selective solvents or solvent combinations such as (a) methyl ethyl ketone; (b) methyl isobutyl ketone; (c) combinations of each of (a) and (b) with benzene or toluene, mixtures of benzene and toluene, or raffinates rich in benzene and toluene; and (d) other selective solvents such as dichloromethane, methylene chloride, etc.
The present invention, however, is not only applicable to, and employable in, solvent refining or solvent dewaxing processes utilizing the foregoing solvents, solvent combinations and/or solvent mixtures, but it is also applicable to, and employable in petroleum deasphalting processes which also use selective solvents or solvent systems such as one or more of the following solvents such as propane, butanes, etc., taken alone or in admixture with traces of ethane, C.sub.5 alkanes, propylene, and butylene in non-deleterious quantities.
Generally, petroleum deasphalting and solvent refining and dewaxing processes are well known in the art and are effected by introducing a material to be treated, that is, a charge stock in which there are present at least two different chemical entities which have different solubilities or preferences in the presence of each other in a solvent into a solvent extraction zone in which the charge stock is contacted with a solvent.
At one end of the zone, in the usual petroleum deasphalting or solvent refining or dewaxing process, there is removed a raffinate-mix fraction which represents that chemical entity which is not as soluble in the solvent as the extract-mix fraction which contains the other entity and is removed at the other end of the zone.
An important aspect of these processes, including the preferred embodiments thereof such as furfural refining, propane or butane deasphalting, or methyl ethyl ketone/toluene dewaxing, is the subsequent treatment of the raffinate-mix, extract-mix fractions and/or the wax-free oil fraction, and/or the wax fraction and/or the asphalt-free oil fraction, and/or the asphalt fraction to recover not only the upgraded petroleum fraction but also to recover substantially all of the solvent from such fractions. Generally, the separation of solvent from the raffinate-mix fraction is readily performed in a single fractionating tower utilizing stripping techniques. Treatment of the extract-mix fraction to recover the solvent therefrom, however, is substantially more difficult because of the greater volume of the solvent in the undesired extracted constituents of the petroleum fraction being treated. Thus, a more complicated recovery scheme is required to separate solvent from the extract fraction. Like problems exist in the treatment of the filtrate in solvent dewaxing or deasphalting processes.
2. Description of the Prior Art
In U.S. Pat. No. 2,895,908 to Beavon, there is disclosed a process for recovering furfural from the extract fraction by successive passage through two vacuum flash towers operated preferably in the absolute pressure range of from 5 to 700 mm. Hg.; it is stated in this patent that it is desirable to operate the first flash tower at an absolute pressure greater than the second flash tower.
In U.S. Pat. No. 2,943,050 to the same patentee, there is disclosed a solvent extraction process, i.e., deasphalting, wherein the oil solvent extraction fraction is successively passed through two flash drums to recover solvent.
The nature of the petroleum fraction being treated, i.e., in terms of the initial boiling points thereof, has also led to the use of such separation techniques as simple distillation, azeotropic distillation, or a combination of extractive distillation and azeotropic distillation, such as disclosed in U.S. Pat. No. 3,167,501 to Woodle.
Although the techniques of solvent extraction have been refined to a sophisticated degree, the methods of solvent recovery, even in plants being presently constructed, are based on technology used twenty years ago. As a result, a considerable portion of the operating cost is borne by the heat energy to distill the solvent. The amount of energy is almost proportional to the total amount of solvent circulated through the extraction with between about 75 to about 85% being used in the distillation of the extract phase. Consequently, the recent energy crisis and the increased consciousness of the world with respect to energy have dictated a necessity to re-evaluate processing technology so as to minimize energy requirements and thereby reduce fuel costs. Thus, new plants need to be designed with technical inovations which minimize heat requirements, and existing plants need to be re-evaluated as well as revamped to reduce energy requirements.